Biaxially stretched and blow-molded bottles made of polyethylene terephthalate (hereinafter referred to as “PET”), the so-called PET bottles, have high levels of transparency, mechanical strength, heat resistance, and gas barrier property, and up to now, have been in wide use as the containers for various beverages. Conventionally, what is called hot filling is utilized as a method of filling the PET bottles with content fluids, e.g., juices, teas, and the like, which require pasteurization. This involves filling the bottle with a content fluid at a temperature of about 90 degrees C., sealing the bottle with a cap, and cooling the bottle. This process causes the pressure inside the bottle to decrease considerably.
As regards the application of use involving hot filling described above, Patent Document D1, for example, teaches that the body is provided with the so-called vacuum absorbing panels, which are, by design, easily deformed into a dented state under a reduced pressure condition. At the time of a decrease in internal pressure, these vacuum absorbing panels perform a vacuum absorbing function by deforming into the dented state, thus allowing the bottle to retain good shape while ensuring that the portions of the bottle other than the vacuum absorbing panels have rigidity enough to avoid troubles on the bottle conveyor lines, during storage in piles, and inside the automatic vending machines.
On the other hand, in some cases it is necessary to avoid forming the vacuum absorbing panels on the body out of regard for the design of bottle appearance. Since the vacuum absorbing panels tend to be subject to flexural deformation, it is also necessary for body walls to have high surface rigidity to give the body high shape retainability enough to be able to stack the bottles on their sides inside the vending machines. For these applications of use, Patent Document D2, for example, shows a synthetic resin bottle which has no vacuum absorbing panel in the body wall, but in which the vacuum absorbing function is performed by the upward drawing deformation of a bottom plate. Especially in the cases of small-size bottles with a capacity of 500 ml, 350 ml or 280 ml, the vacuum absorbing panels disposed in the body wall would have a limited panel area. In that case, it would be difficult to fully satisfy both of the vacuum-absorbing function and the rigidity or buckling strength of the body. Therefore, the vacuum-absorbing function need be performed by the deformation of bottom plate as described above.
As an example, FIG. 4 shows a bottle 101 in which the vacuum absorbing function is performed by a bottom plate of a bottom 105, which plate deforms so as to draw upward. FIG. 4(a) is a front view; and FIG. 4(b) is a bottom view. The bottle 101 comprises a body 104 having a thick wall and peripheral groove ribs 107 to give high surface rigidity and high buckling strength to the body 104.
The bottom 105 comprises a ground contact portion 116 disposed at the foot of an outermost peripheral wall and successively connected to the body 104, a ring groove 115 disposed on the inside of the ground contact portion 116, a flat ring portion 113 disposed on the inside of, and integrally connected to, the ring groove 115, and a central concave portion 112 disposed at a center of the bottom 105 and successively connected to the flat ring portion 113. When there is a decrease in the pressure inside the bottle 101, the body 104 keeps its shape, but the ring groove 115 deforms starting from the base of an inner peripheral wall of the ground contact portion 116. Then, the flat ring portion 113 and the central concave portion 112 draw upward (i.e., they deform in the arrowed direction in FIG. 4(a)) so that the bottle performs the vacuum absorbing function.